After a bit of miscommunication, my wife and I ended up buying 6 litres of milk between us. I don't really want to waste it, so I'm wondering if it's safe to freeze some of it and defrost and use it later?

The density of ice is around .92 g/mL, so you need to save an absolute minimum of 8% of the space in your container for expansion, 80 mL per liter. (For my fellow Americans coming across this, for a gallon, that's 1.3 cups. See why we should use metric?) Personally, I'd say better safe than sorry, and double that.
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Jefromi♦Oct 29 '10 at 16:39

@Jefromi, you just need the container to have enough airspace and flexibility. No amount of airspace would be enough for me to put a glass bottle in the freezer, but paper cartons can go in straight from the store.
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RBerteigNov 1 '10 at 7:17

@RBerteig: I was just trying to quantify "enough airspace" by giving a clear upper bound.
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Jefromi♦Nov 1 '10 at 7:48

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@Jefromi, I didn't really intend to criticize the calculation. It is a good idea to have an upper bound in mind. I'd still be careful about glass bottles in freezers, though. Even though paper is usually fine, I'd inspect it for burst seams before thawing, as that saves a cleanup...
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RBerteigNov 1 '10 at 7:59

@RBerteig: No worries. The extra advice on material is good.
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Jefromi♦Nov 1 '10 at 13:37

Higher fat content milk can separate more, but homogenization seems to reduce the effect. If the milk does separate, it can be shaken or whipped (or used in baking if neither of those work).
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Bruce AldersonOct 29 '10 at 16:07

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@sdg, tickled by the first line in your answer. hope you don't mind. Cottaging has a different meaning here in the UK.
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Tea DrinkerOct 29 '10 at 19:52

@Tea - ok, you have me intrigued... Care to share? Or can you on a family cooking site? :-)
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sdgOct 31 '10 at 0:15

@Tea Drinker, That is a prime example of two cultures separated by a common language. ;-) As for frozen milk, I remember freezing it part way deliberately when I was a kid. Milk was good just at the point where crystals started to form. It also helped make low-fat milk taste better by removing some of the water.
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RBerteigNov 1 '10 at 7:15

We always freeze milk and have never had a problem. We just freeze it in the plastic bottle it came in (the kind you get from supermarkets in the UK). I've not tried freezing glass bottles or plastic cartons.

I buy 3-6 gallons of milk and freeze them. I usually get half whole and half 2%; the whole milk seems to separate a bit, but one or two firm shakes after it's defrosted seems to do the trick. The milk does acquire a strange transparent yellow color, but upon defrosting is just fine. I've never had to pour any milk out to make room for expansion, as long as the jug itself is not damaged. Paper cartons work just as well for freezing; I've never had one burst.

In the old days of the paper cartons for half gallons of milk (before they put the little plastic screw-on top on 'em), my mom would do it all the time ... I seem to recall almost a whole shelf of our stand-up freezer being dedicated to milk storage. She'd just put it in the freezer, in the carton ... but I can't remember how she thawed it ... likely overnight in the fridge, as I don't remember seeing it.

We typically got 2% milk ... I have no idea if that makes a difference or not.

I buy a gallon of milk (2%) at a time. Less shopping to do. Living alone, I can't use it all up at one time. For years I have been filling cleaned snapple size bottles about 80% full and standing them up to freeze on my freezer door. About 24-36 hours before I need milk, I take out a bottle and put it in the fridge. I've never had a broken bottle.